[2.1.x] Fixed #29749 -- Made the migrations loader ignore files starting with a tilde or underscore.

Regression in 29150d5da8.
Backport of 32fbccab40 from master

tests/migrations/test_migrations_private/0001_initial.py

@@ -0,0 +1,5 @@
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+    pass